Natural rubber is the ideal elastomer for producing adhesive tapes provided high ageing resistance is not a requirement. Particular advantages are
the low price of the elastomer
the low price of suitable tackifiers (resins and so on)
the good balance between the physical properties such as unrolling force, shear strength, tack and bond strength.
To date it has proved possible to process natural rubber only from solution. There has been no lack of attempts to process natural rubber as an aqueous system as well. Technical realization fails owing to the extreme shear sensitivity of the latex; only by adding fairly large amounts of surface-active substances or by blending with other latices is it possible to achieve a certain measure of processability, and then only at the expense of a severe loss in the target profile of properties.
Until now, the processing of natural rubber compositions from the melt was impossible. If the preparation of hot melt PSA (pressure-sensitive adhesive) compositions is attempted, then homogenization necessitates strong and lengthy shearing. This causes severe degradation and hence a loss in performance of the adhesive tape, despite which it is impossible to wholly avoid specks of rubber. Improving the homogeneity by shearing (masticating) the rubber, then preparing a batch with a portion of the tackifier and subsequently processing this batch in a further step with the addition of further raw materials to the adhesive composition has been tried: this technique is described in WO 94/11175. WO 95/25774 is further evidence that the processing of non-thermoplastic material is possible only with mastication, which has the deleterious effect of degrading the polymer. A further disadvantage of all process trials with solid rubber is the necessity of batch operation, which drives up the production and quality control costs. Adhesive compositions of this kind have so far been used only to manufacture low-grade cloth adhesive tapes (duct tapes). Because of these problems, thermoplastic elastomers (styrene-diene block copolymers) have been developed which go a long way towards avoiding these drawbacks. However, they result in product properties markedly different from those of natural rubber. For instance, it has so far been impossible in this way to produce high quality products such as readily redetachable film adhesive tapes or paper-based adhesive tapes for painting. A significant defect of such adhesive compositions comprising thermoplastic rubbers is their low heat resistance. Ageing resistance is likewise much less favourable than in the case of natural rubber adhesive compositions. Furthermore, in terms both of the elastomer and of the suitable tackifiers, such formulations are significantly more expensive than those based on natural rubber.
Natural rubber is produced as latex. The commercial solid rubber is obtained from latex by precipitation. Without the use of solvent it is impossible to mix solid rubber in the form of bales or pellets with tackifiers, fillers, antioxidants, plasticizers, etc. with complete homogeneity and above all to avoid the associated shearing and degradation of the elastomer. If the latex is mixed homogeneously with a dispersion of these additives, the problems of both homogeneous mixing of the components and shearing and degradation of the elastomer disappear. Unfortunately, such water-based adhesive compositions cannot be used in practice for coating on production lines owing to their extreme coagulation sensitivity: even during processing, gel particles, or complete coagulation, occur.
Process of the Invention
When natural rubber latex, melted or finely particulate tackifiers and any further components are mixed in a mixer, compounder, extruder or the like, the mixture is surprisingly homogeneous, because the latex mixture does not coagulate before the end of complete addition and thorough mixing. In the specific case of preparation, it is advantageous to make a correct choice of the sequence and amount of the components and not to subject the mixture to any more shear stress than is necessary, since under adverse conditions it is possible to provoke premature coagulation. The stability can be raised by adding water, ammonia, alkali metal hydroxide solutions, or surface-active substances such as emulsifiers. In batch operation the best thing is generally not to add the latex and the other components in alternation but instead either to introduce all of the latex at the start and then to mix in the remaining components or to introduce these components as a mixture at the start and then to add all of the latex. If the latex is introduced at the start, then the first addition should be the plasticizer (if required); this applies in particular to mineral oils or liquid hydrocarbon resins. In this way it is easy to mix all of the desired components before the rubber solidifies through coagulation. This is highly surprising, since not only is the latex shear-sensitive when stirred thoroughly but to the skilled worker a mixture which has a solids content of 80% or more and yet can still be stirred is unimaginable.
Depending on formulation, the adhesive compositions of the invention are still liquid and hence can be processed directly using an applicator unit, ball together tackily, or form a non-tacky and readily conveyable solid. In general there is no discernible separation (e.g. emergence of the aqueous phase). When using Hercolyn D-E, for example, liquid mixtures with a water content of from 10 to 50% by weight are produced which can be used for coating but require subsequent drying after the coating operation. The use of very high proportions of fillers or pigments results in solid mixtures which can be conveyed without tack. These solid or liquid mixtures can be supplied, for example, to a twin-screw extruder or planetary gear extruder, which removes the residual water in a vent section and then feeds a coating die or roll-type applicator unit. The applicator unit then coats the substrates directly or indirectly (for example, coating onto a roller or liner and transfer to the substrate by lamination) or applies the composition to a liner, without a backing. Dewatering can also be achieved by means of heat or subatmospheric pressure in a stirring unit, such as compounders or continuous mixers; internal mixers (Banbury) are less suitable because it is more difficult to remove the adhesive composition. The compositions (unless highly filled or pigmented) have a pressure-sensitive tack even without compounding; in other words, diffusion is sufficient to allow molecular penetration of rubber and resin. This shows how easy it is, in the manner of the invention, to achieve homogeneity without the need for extreme shearing as in the case of solid rubber. Prolonged compounding in batch operation is therefore unnecessary; just the short residence time in an extruder is sufficient, so enabling the manufacturing process to be continuous. It is possible to carry out the mixing of the latex with the other components and the dewatering in one extruder. The resin may, for example, be passed through a Condux mill downstream of the weigh feeder and then onto the feed zone; alternatively, the resin melt can be pumped in at just under 100xc2x0 C.
Because of the short residence times when the adhesive composition is sheared, the rubber undergoes little if any degradation. The adhesive compositions of the invention therefore have K values of in particular xe2x89xa780, preferably xe2x89xa7125 and, with particular preference, xe2x89xa7140, which give rise to good shear strengths. Even K values of about 160 are possible for adhesive compositions, whose shear strength far exceeds that even of good conventional solvent-based compositions. The K values of conventional solvent-based compositions lie approximately between 130 and 150, but may even be considerably lower if CV rubber is used. A particularly surprising fact was that the compositions of the invention, with such high K values, can be used for coating without problems. It is supposed that the rubber particles (of the former latex droplets) absorb resin and plasticizer by swelling, without the rubber particles undergoing full mutual penetration in the process, and owing to the entanglement the viscosity rises as sharply as expected from the K value. By adding rubber latices with low Mooney values (as used, for instance, to prepare CV 50 with a Mooney value of 50) the K values of the compositions can be regulated further, so enabling the processing properties and adhesion properties to be matched to requirements.
The coating thickness of the adhesive compositions can be in particular between 2 g/m2 and 3 kg/m2. Depending on application and backing, coating thicknesses of from 12 to 500 g/m2 are particularly suitable, preferably between 20 and 200 g/m2.
Since with this process it is possible to obtain shear strengths varying from adequate to outstanding, additional crosslinking is unnecessary. In any given case it may be sensible to carry out crosslinking, for example with gamma rays, ultraviolet radiation or electron beams, or with crosslinking resins. This is possible with high-temperature applications such as for adhesive tapes for painting motor vehicles or when using rubber latex with a very low Mooney viscosity. Since irradiation may damage the backing the dose should be kept as low as possible. The radiation dose can be reduced by using photoinitiators or by adding esters of allyl alcohol, of methacrylic acid or of acrylic acid to the adhesive composition.
Raw Materials
For the adhesive composition it is possible in addition to rubber and tackifiers to use additives as well, such as fillers, antioxidants, plasticizers or pigments, as are described, for example, in Handbook of Pressure Sensitive Adhesive Technology, ed. Donatas Satas, N.Y. 1989, or rubber additives such as lubricants and masticating agents.
The known natural rubber latices are suitable for the process of the invention and for the adhesive tapes produced thereby. It is advisable to filter the latex before processing. The natural rubber latices concerned have been concentrated, chemically treated and/or treated with additives; for further details see The Natural Rubber Formulatory and Property Index, ed. Malaysian Rubber Producers""Research Association, 1984, Ullmann""s Encyclopedia of Chemical Industry, Vol. 23, p. 221 ff., VCH Weinheim 1993, and Encyclopedia of Polymer Science and Engineering, Vol. 14, page 687 ff., J. Wiley and Sons, 1988. The use of untreated fresh latex, although possible, is fairly impracticable owing to the high transportation costs. Therefore, concentrated latices are preferred. The commonest concentration technique is centrifugation. Such a technique, through the reduction of non-rubber constituents, is particularly suitable for adhesive compositions. The use of special grades with a particularly low protein content (DPNR=deproteinized rubber) is possible but not rational on economic grounds. Revertex latex prepared by concentrated evaporation is suitable; experiments have indicated that for use in accordance with the invention it should be diluted with water to about 60%. It is less sensitive to shearing than is centrifuge latex, but the bond strengths of the adhesive compositions produced from it are in some cases lower than those of similar compositions prepared using centrifuged latex. Also suitable are latices prepared by creaming (for example, by treatment with ammonium alginate) or electrodecantation. For use in accordance with the invention, preference is given to the use of centrifuge latex. A grade which has been found to be particularly suitable is high ammonia (HA) or full ammonia with 0.5-0.7% ammonia. The grade LA-TZ (low ammonia with about 0.2% ammonia and thiuram stabilization) is almost as suitable but is slightly more sensitive in processing. To control the molecular weight of the rubber the natural latex can be treated with chemicals such as hydroxylamine hydrochloride, which reduces the shear strength of the adhesive composition but has a positive effect on its processing viscosity. A pale colour can be obtained by treatment with bleaches such as sodium hydrogen sulphite. SP/PA grades (superior processed rubber with prevulcanization) such as Revultex can additionally be used, and lead to a higher shear strength of the adhesive composition. OENR (oil-extended natural rubber) is an alternative to the admixture of plasticizers in the process of the invention. There are certain latices comprising synthetically prepared isoprene rubbers which are intended to fall within the designation natural rubber latex for the purposes of the patent. These are, for example, latices of high-cis-polyisoprene, or natural rubber which has been epoxidized or grafted (for example, with methyl methacrylate, such as Hevea Plus MG 49).
In addition to natural rubber latex it is also possible in addition to use one or more synthetic latices based, for example, on SBR, cSBR, BAN, NBR, EVAc, SBS, SEBS, SIS, PU, BR, IIR, ACM or XIIR.
In addition to natural rubber latex it is also possible in addition to use one or more semi-liquid to solid elastomers. These are, for example, SBR, cSBR, BAN, NBR, EVAc, SBS, SEBS, SIS, PU, BR, IIR, ACM or XIIR or natural rubbers such as crepe, SMRL, CV 40, CV 50, and chemically or mechanically degraded types such as Lorival(copyright) or liquid synthetic polyisoprenes. Their proportion must not of course be too high, especially in the case of grades having high Mooney viscosity, since otherwise the problems described at the outset occur. If a natural rubber latex PSA composition comprising conventional rubber is modified by natural rubber latex, it is possible to reduce the drawbacks such as high melt viscosity and low shear strength of the adhesive.
Examples of suitable tackifiers are hydrocarbon resins formed, for example, from unsaturated C5 or C7 monomers, terpene-phenolic resins, terpene resins formed from raw materials such as xcex1- or xcex2-pinene, rosin and its derivatives, such as disproportionated, dimerized or esterified resins, in which context it is possible to employ glycols, glycerol or pentaerythritol, and others, as listed in Ullmanns Enzyklopatdie der technischen Chemie, Volume 12, pp. 525-555 (4th ed.), Weinheim.
Examples of suitable fillers and pigments are carbon black, titanium dioxide, calcium carbonate, zinc carbonate, zinc oxide, silicates, silica, metal powders, and also solid and hollow beads of glass, polymer or ceramic.
Examples of suitable plasticizers are aliphatic, cycloaliphatic and aromatic mineral oils, liquid rubbers (e.g. nitrile, polybutadiene or polyisoprene rubbers), liquid polymers of butene and/or isobutene, acrolates, polyvinyl ethers, liquid resins and soft resins based on the raw materials for tackifier resins, lanolin, and other waxes or liquid silicones.
Examples of suitable crosslinking resins are phenolic resins or halogenated phenolic resins, melamine and formaldehyde resins, maleimides and radiation-curable agents such as triallyl cyanurate, and polyfunctional esters of acrylic and methacrylic acid.
Backings
Examples of customary films suitable for coating for adhesive tapes are those comprising polypropylene, especially monoaxially and biaxially oriented polypropylene, polyethylene, polyesters such as PEN and PET, rigid and plasticized PVC, polyimide, or metal foils, and also films prepared from such materials and other raw materials by coextrusion or mixing.
Also suitable for coating are nonwovens of, for example, polypropylene, polyester, polyamide or cellulosic fibres.
Papers and wovens are particularly suitable, since they have especially good heat stability and tensile strength and hence can also be processed using a twin-roll applicator unit. Examples of papers suitable for coating are those comprising natural fibres, such as cellulose, but also those using synthetic fibres, which are preferably impregnated with, for example, NBR, SBR, polyurethane latex, PVD latex or acrylate latex, alone or in combination with paper softeners such as polyols and their derivatives or reactive resins (based on formaldehyde, for example). Particularly suitable papers are those having an elongation of break of  greater than 5%, which is achieved by creping and other manufacturing techniques, which are particularly suitable for bonding on curved surfaces.
Wovens are also suitable for coating and may consist, for example, of synthetic fibres such as rayon, polyamide or polyester, or of natural fibres such as cotton or hemp.
The backings can also be laminates prepared by lamination using materials referred to above. The backings may also be modified by radiation treatment.
All of the said backings can be provided with surface treatments. Examples of such treatments are, to promote adhesion, corona, flame or plasma treatments, coatings of solutions or dispersions, or liquid radiation-curable materials. Further possible coatings are printed coatings and anti-adhesion coatings, examples being those of crosslinked silicones, acrylates (e.g. Primal(trademark) 205), polymers with vinylidene chloride or vinyl chloride as monomer, or stearyl compounds, such as polyvinyl stearylcarbamate, chromium stearate complexes (e.g. Quilon(trademark) C), or reaction products of maleic anhydride copolymers and stearylamine.
The term backings here is also intended to refer to release films and release papers which are suitable for provisionally carrying the adhesive composition before its transfer to the desired substrate in a subsequent step.
Test Methods
The results of measurement given in the examples were determined under standard conditions in accordance with DIN 50014-23/50- Part 1. The bond strength to steel or the reverse of the backing was tested in accordance with PSTC 1. The application rate was determined by differential weighing, after washing off the adhesive composition with hexane. The shear strength (holding power) was determined essentially in accordance with PSTC 7 at 23xc2x0 C. but with variations: method A with a load of 10 N and a bond area 20 mm long and 13 mm wide, method B with a load of 20 N and a bond area 20 mm long and 13 mm wide, and method C with a load of 10 N and a bond area 25 mm long and 25 mm wide.
The rubber was characterized using the K value and the Mooney viscosity. Calculation of the K value from the relative viscosity is described in Fikentscher, Cellulose-Chemie 13 (1932), p. 58 ff. and Polymer 8 (1967), p. 381 ff. The viscosity of a 1% strength solution of the natural rubber in toluene is measured in a Vogel-Ossag viscometer in accordance with DIN 51561. Prior to dissolution in toluene, the latex was dried in a thin film on a silicone paper at 90xc2x0 C. In the case of adhesive composition the initial weight was chosen so as to give a calculated 1% of rubber in the toluene (the other constituents of the adhesive composition are reckoned as solvent). The solutions were filtered before measurement. The Mooney viscosity was tested in accordance with ASTM D 1646.